Treatment of carbon fibers

ABSTRACT

A method of treating high modulus, high strength carbon fiber to improve its bonding characteristics in a resin matrix comprising immersing the fiber in concentrated nitric acid for 4- 8 hours at refluxing temperature.

United States Patent Scola et a1.

[54] TREATMENT OF CARBON FIBERS [72] Inventors: Daniel A. Scola,Glastonbury; Hilton A.

Roth, Cheshire, both of Conn.

United Aircraft Corporation, East Hartford, Conn.

[22] Filed: June 18,1970

[21] Appl.No.: 47,490

[73] Assignee:

[52] U.S.Cl. ..l17/47R,23/209.1,8/115.5,

8/140, 260/37, 264/D1G. 19, 117/161 P, 117/161 1 Z8 51 111:; C1. ..C0lb31/07, 844d 1/092 [58] Field ofSearch. ...23/209.1;8/1l5.5,115.6,140;117/47 R, 47 H, 106 R, 118,228; 264/DIG. 19;

[56] References Cited UNITED STATES PATENTS 2,615,932 10/1952 Marko eta1 ..117/47 R 2,669,598 2/1954 Marko et a1 ..1 17/47 R java [1513,660,140 51 May 2,1972

3,529,934 9/1970 I Shindo ..23/209.l 3,294,572 12/1966 Piccione ...117/47 R 3,550,247 12/1970 Evanset a1 ..117/228 FOREIGN PATENTS ORAPPLICATIONS 438,995 6/1964 Japan .23/209.1

OTHER PUBLICATIONS Derwent Japanese Textiles V01. 7, No. 15, pg.6-tit1ed Carbon Fibres.

Chemical Abstraets V01. 64 p. 12862 c (1966).

Primary ExaminerWi11iam D. Martin Assistant Examiner-Michael SofocleousAttorney-John D. Del Ponti [5 7] ABSTRACT A method of treating highmodulus, high strength carbon fiber to improve its bondingcharacteristics in a resin matrix comprising immersing the fiber inconcentrated nitric acid for 4-8 hours at refluxing temperature.

8 Claims, 4 Drawing Figures 1 TREATMENT or CARBON FIBERS BACKGROUND OFTHE INVENTION resins such as thou suitable for use in applications ofthe aerospace industry, e.g. the epoxy or polyamide resins. The bond ofsuch fibers to the resins has characteristically been poor typicallyyielding graphite fiber-resin matrix composites with low shearstrengths; generally in the range of 3',500-4,500 psi for low fibercontent composites (-45 vol 20 and below 3,500 psi for high fibercontent composites (45-65 vol%).

While it has been suggested to surface treat carbon fibers in order toimprove their shear strength by various methods,

such as for example, by oxidation, the resulting degradation of 2 5other fiber properties, especially tensile strength, has been a problem.

SUMMARY OF THE INVENTION In accordance with this'invention carbon fiberis exposed toboiling nitric acid by immersion therein for an extendedperiod of 4-8 hours. Carbon-resin composites fabricated with carbonfibers treated according to the present invention exhibited a highresistance to shear failure while maintaining high flexural strengths.

An understanding of the invention will become more apparent to thoseskilled in the art by reference to the following detailed descriptionwhen viewed in light of the accompanying drawings, wherein:

FIG. 1 is a graph illustrating the effect of contact time on compositeshort beam shear strength;

FIG. 2 is a graph illustrating the relation between fiber volume andcomposite short beam shear strength;

FIG. 3 is a graph illustrating the relation between fiber volume andcomposite flexural strength; and

FIG. 4 is a graph showing the correlation between specific surface areaand short beam shear and transverse tensile strength.

DESCRIPTION OF THE PREFERRED EMBODIMENT The technique of treating carbonfibers according to the present invention comprises immersing carbonfibers in boiling, concentrated nitric acid for a period of timesufficient to activate the fiber surface with little or no concomitantdegradation in fiber properties.

The process was performed on a batch basis by winding the yarn onto aspool and lowering it into an acid filled resin kettle which wasprovided with a reflux condenser and which served as the reactionvessel. The spool was comprised of two series of parallel rodsconcentrically arranged at two selected radial distances from a centralrod, all rods being coated with Teflon and supported between two Tefloncoated hubs. The carbon yam was wound about each series of parallel rodsso that there existed a space between the yarn layers. After the nitricacid treatment, the yarn was rinse cycled by rinsing in distilled waterfor three 15 minute periods, rinsing in a dilute solution of ammoniumhydroxide for 10 minutes, rinsing in distilled water and then rinsing inacetone for two 15 minute periods. After the rinse-cycling the yarn wasair dried'in a hood and finally dried in an air-circulating oven at 100C for 15 minutes, impregnated with resin by passage therethrough andwound onto a drum in tape form and processed into a com- 7 posite.

During experimentation, commerciallyavailable Thornel I 50 yarmI'IitcoHMO-50 and Morganite I yarns were utilized with conditions and resultingproperties as shown in'lables I and Il.

TABLE I.-GRAPI-IITE FIBER, EPOXY RESIN COMPOSITES Short beam Flexurnlproperties shear (avg) strength, Fillet Composite p.s.i. Strength,Modulus, content. Density, Number Yarn (size) 'Ireatment 8/1) 5/1 10p.s.i. 10 psi. 0 g. c. T-EO (H2O) None 3, 760 60.1 11. 3 83 1. 370 T- (H0) (I 3, 780 (i3. 0 ii. (i 37 l. 33'. T-50 (H2O) 2, 080 03. l 16. 3 -18I. 430 l50 (H O) 2, 600 70. 2 30. ii 0'. I. 475 T-50 (PVA) 3, 760 03. 813.0 30 1. 410 T-50 (PVA) 4, 250 74. 0 10.0 12 I. 390 'I50 (PVA) l, 72080. 1 l7. 3 5-1 I. 416 'l50 (PVA) 7, 058 115. 3 .10. 7 53 1. 460 'I50(H20) 7, 750 7!). 4 12. 0 31 1. 356 T450 (20) ii, 500 78. 2 ll. 5 37 1.350 'I50 (IVA) 7, 7-10 107. 3 l7. 7 52 I. 405 'l50 (PVA H, 004 108.0 27.ii 52 I. 408 'l50 (I'VA) 7, 450 101. 5 31. 8 48 1. 450 I-50 (IVA) .do 7,600 I01. 5 20. 5 54 1. 472 MG-50 (none) ][N()Il, 4 hrs. rellux. 8,240105.0 21.7 41 1. 400 (Io 70%'IIN( 0111's. l'eIluX 8, 200 104. 8 32. ii43. (i I. 430 do... 70% lINOs, 8 hrs. rellux. 8, 500 106. {I 24.1 40.4 1. 375 .do. 70%I1N03, 10 hrs. reflux. 8, 300 110.0 25.1 45.1 1. 435All). 70% HNOa, 8111's. reflux. 0 770 126. 7 28. 7 00. 4 I. 544 (Io tl07, 250 I35. 1 27.2 [53.5 1.523 ...(l0 0,420 124.8 02.0 1.508 .110 .(107,140 120,3 20,0 02.0 1.526 .1) (It) J, .130 7. li 213.5 53. 7 1. 11 (In00.. H, lilill 128.1 15,0 51. 5 l. 480 J5 do NOllt. 1, M0, 70. -l .'-l.!51.11 I. -12" .fti l\ lur .',nui'lu l (unlrrnlrtl) 7(l,',, "N01, 8 hrs.rvllux 7,515 00.3 l) l.-lli|i .7 Noll! 11,530 lJ. J 1.571)

10 l 0'14 ne o1.

TABLE II 1 lclylmidc/graphlte composites made with nitric acid treatedfiber (8 hours) Q I Shear Flexural Flexual lolyimide I Density, strengthstrength modulus, rosin Fiber (v/o) g./co.- (p.s.i.) (K s. i.) b 10p.s.l

lI-703 '1horncl-50 (52.5 1.45 moo 00. 1 22 7 lI-700 MG-50 (e1). 1. an 7,500 121, u 28. 4 PI-703- HMG50 (55).. 1.53 7,450 116. 25.5 PIT-703HMG-50 (55) uni-t outed. l. 52 5, 000 63. (l 24. 0 PIT-700 HMO-50 (57)untreated" 1. 54 4, 880 81. 8 21. 0 PI703.. Thornel-50 (50) untreated...l. 43 4-, 000 r 60. 0-

I Span-to-depth 1 6/1. b 4- polnt flex test.

Further tests, wherein the tensile strengths of treated and untreatedyarns were made are shown in Table III.

TABLE ill 1 l'Ir-nsile strengths of untreated and treated graphiteyarns] I Yarn strength p.s.i.)

UARL tested Before After I treattreat- Percent Yarn type Surfacetreatment merit ment change 'lhorncl-50 (PVA) 70% IINO 8 hrs., 181 188+3. 9

lot #06238'1415. 120 C. '1hornol-50 (PVA) -.do 254 249 1.9

lot #09208T-3E. 'lhornel-50 (IVA) "do"... 179 213 +16 lot #06228T-3W.Illtqo l-IMG-so .(lo 219 200 8. 7

lot #C-07158-10A llitco HMO--50 'do v 239 232 3. 0

lot #C-12l08-1. llltco HMGtO' 70% HNO;, 8 hrs., 23%! .230 -3. 8

lot #C-12l08 1. 120 0., then- NH OHwash.

In FIG. I, the effect of various contact times of carbon fiber with 70%HNO at 120 C on short beam shear strength of a composite having a2,256-0820 epoxy resin matrix is shown.-

lt can be seen, as a result of testing, that in order to achievesignificant shear strength improvement, a minimum of 4 hours contacttime is necessary. Further it can be seen that contact times of greaterthan 8 hours while not detrimental, produce no significant increaseinshear strength.

FIGS. 2 and 3 show the comparison between untreated fibers and thosetreated according to the present invention with respect to the effectsof fiber volume on shear and flexural strength. In each case, thetreated fibers display a significant increase'in strength regardless ofvolume fraction.

It is believed that the great improvement of bonding characteristics ofthe carbon yarns treated according to the present nitric acidtreatmentprocess are due primarily to the increase inspecific surfacearea and in the increase of surface reactivity caused by the treatment.Increasing the exposure of the fiber causes a steady increase in thesurface area with a very gradual increase in'shear and transversetensile strength. The results in Table lV'below, for example, indicatethat there exists a definite correlation between the surface properties(specific surface area and the concentration of acid sites per unitarea'of fiber based on sodium hydroxide adsorption) and the observedincreases in shear strength. in the treatment, it has been found thatthe number of acid sites per unit area increases initially and thenlevels off and diminishes. in the particular experiment illustrated byTable IV,.it can be seen that while the concentration of acid sites/unitarea falls off as the surface area increases to 24 mlg, it is stillgreater than the unlIitcO lulu-50 graphite yarn-=1mlunirlirrt-tionnlll.\ltl 5r) epoxy resin composites 1! b H Compositeproperties Fiberproperties 1 Short Specific NaOI-I NaOH beam surface adsorbed adsorbedshear I .Flber area, mole/g. mole/m. strength volume, Treatment m. /g.fiber fiber of p.s.i. percent None 0.87 0. 006 0. 074 4, 490 64 HNOG oxition (4 hr.) 3. 4 2. 04 0. 5, 600 64 HNO oxidation (8 hr.) 7. 3 3.0 0.41 6, 090 5i) HN O oxidation (12 hr.) 10. 5 3. 60 t). 34 U, 500 (55IINO3 oxidation (18 hr.) 24. 0 5. 9 0. 25 7,000 64 What is claimed is: I4 1; A method for improving the bonding=characteristicsof high strength,high modulus carbon fiber with resin matrix material withoutsignificantly degrading the mechanical properties ofthe fiber'whichcomprises, prior to impregnating the carbonfibers with said resin matrixmaterial, immersing said carbon fibers in concentrated nitric acid atrefluxing temperature for at least 4 hours.

2. The method of claim 1 wherein from 4 through 8 hours.

3. The method of claim 2 wherein the acid is nitric acid.

4. A method for improving the bonding characteristics of high strength,high modulus carbon fibers with resin matrix material withoutsignificantly degrading the mechanical properties of the fibers whichcomprises, prior to impregnating the carbon fibers with said matrixmaterial, subjecting said fibers to the action of concentrated nitricacid at refluxing temperature for a period of time sufficient toincrease the specific surface area of the fibers to 3.4-7.3 m lg.

5. A method for improving the bonding characteristics of high strength,high modulus carbon fibers with resin matrix material withoutsignificantly degrading the mechanical properties of the fibers whichcomprises, prior to impregnating said carbon fibers with said resinmatrix material, exposing said fibers to the actionof concentratednitric acid at refluxing temperature to cause an increase in the numberof active sites the fiber is immersed at the fiber surface as measuredby NaOH absorbed per unit area and continuing the exposure of the fiberto the acid at treated fiber. It thus appears that both the increase insurface 7 area and in surface reactivity contribute to the improvement Ileast until the number of active sites at the fiber surface ceases toincrease. I

. 6. A method for the production of a carbon filament-resin compositeshaving a high resistance to shear failure while maintaining highflexural strength comprising, exposing high modulus, high strengthcarbon filaments to concentrated nitric acid at refluxing temperaturefor atleast 4 hours, and impregnating said treated carbon filaments in aresin matrix.

7. The method of claim 6 wherein said filaments are exposed for 4-8hours.

8. The method of claim 6 wherein said resin is epoxy or polyimide resin.

2. The method of claim 1 wherein the fiber is immersed from 4 through 8hours.
 3. The method of claim 2 wherein the acid is 70% nitric acid. 4.A method for improving the bonding characteristics of high strength,high modulus carbon fibers with resin matrix material withoutsignificantly degrading the mechanical properties of the fibers whichcomprises, prior to impregnating the carbon fibers with said matrixmaterial, subjecting said fibers to the action of concentrated nitricacid at refluxing temperature for a period of time sufficient toincrease the specific surface area of the fibers to 3.4-7.3 m2/g.
 5. Amethod for improving the bonding characteristics of high strength, highmodulus carbon fibers with resin matrix material without significantlydegrading the mechanical properties of the fibers which comprises, priorto impregnating said carbon fibers with said resin matrix material,exposing said fibers to the action of concentrated nitric acid atrefluxing temperature to cause an increase in the number of active sitesat the fiber surface as measured by NaOH absorbed per unit area andcontinuing the exposure of the fiber to the acid at least until thenumber of active sites at the fiber surface ceases to increase.
 6. Amethod for the production of a carbon filament-resin composites having ahigh resistance to shear failure while maintaining high flexuralstrength comprising, exposing high modulus, high strength carbonfilaments to concentrated nitric acid at refluxing temperature for atleast 4 hours, and impregnating said treated carbon filaments in a resinmatrix.
 7. The method of claim 6 wherein said filaments are exposed for4-8 hours.
 8. The method of claim 6 wherein said resin is epoxy orpolyimide resin.